Provision may be made for a filter which traps particulate matter (hereinafter also referred to as PM) in an exhaust gas. The PM trapped by this filter can be removed by raising the temperature of the filter as well as increasing the concentration of oxygen in the exhaust gas. Removal of the PM from the filter in this manner is referred to as regeneration of the filter. Then, there has been known a technology which enhances efficiency at the time of raising the temperature of a filter by prohibiting regeneration of the filter when ambient air temperature is equal to or less than a predetermined value (for example, refer to a first patent literature).
In addition, there has been known a technology in which at the time of regeneration of a filter, NOx is supplied to the filter from an adsorbent for NOx which is provided at the upstream side of the filter (for example, refer to a second patent literature). In this technology, oxidation of the PM is promoted by NOx. Also, a necessary amount of NOx is made to adsorb to the NOx adsorbent in advance before the regeneration of the filter.
Moreover, there has been known a technology in which a three-way catalyst is provided at the downstream side of a filter, so that the air fuel ratio of an exhaust gas is adjusted to be in the vicinity of a stoichiometric air fuel ratio at which NOx is able to be removed or reduced in the three-way catalyst, after which the air fuel ratio is adjusted to be a lean air fuel ratio with which the filter is able to be regenerated (for example, refer to a third patent literature).
However, an NOx storage reduction catalyst (hereinafter, also referred to as an NSR catalyst) and an NOx selective reduction catalyst (hereinafter, also referred to as an SCR catalyst) may be provided at the downstream side of a filter which traps particulate matter in exhaust gas. This NSR catalyst serves to occlude or store NOx contained in the incoming exhaust gas when the oxygen concentration of the exhaust gas is high, and to reduce the occluded or stored NOx when the oxygen concentration of the incoming exhaust gas becomes low and when a reducing agent exists. In addition, this SCR catalyst is a catalyst which serves to carry out selective reduction of NOx by means of the reducing agent.
Here, at the time of regeneration of the filter, it is necessary to make high the concentration of oxygen in the exhaust gas which flows into the filter. In addition, it is also necessary to make the temperature of the filter high. In such a state, it becomes difficult for the NSR catalyst to store NOx. Moreover, it becomes difficult to supply the reducing agent to the SCR catalyst. Accordingly, there is a fear that at the time of regeneration of the filter, the removal or reduction rate of NOx may decrease.